The Kalends, the Nones, and the Ides

The Kalends (Kalendae), which was always the first day of the month, was sacred to Iuno. The first of the month, following the lunar part of the calendar's operation, was the day following the appearance of the New Moon. On this day, the Rex Sacrorum, together with a pontifex minor, offered a sacrifice to Iuno, and announced, by edict, the date of the coming Nones, which fell on the day after the First Quarter.

The Nones (Nonae), falling on either the fifth or seventh day of the month, came the day after the First Quarter of the lunar cycle. On this day, the Regina Sacrorum offered to Iuno at the Regia, after which the feriae for the entire month were announced. Before the Nones of any given month (with the exception of the Poplifugium), no feriae publicae were held.

Lastly, the Ides (Idus) came, either on the thirteenth or fifteenth day of the month, on the day after the full moon. On this day, a sacrifice was given to Iuppiter, for which reason it is commonly noted as Feriae Iovi in the ancient Fasti. From here, the Pontifices determined the number of days remaining until the next New Moon, which would restart the cycle.

The Ides, also, were always dies Nefasti Publici, given their permanent status as Feriae Iovi, without exception.

Special days in the calendar

These are the rules which are pronounced by the pontifices, and breaking them is nefas (though it can sometimes be expiated).

these days are ill-omened to begin any new project, and any new project would necessarily begin by performing a rite calling for the assistance of the gods. Such religious rites, beginning something new, are not to be performed.

Normal work would still be performed on dies atri, and as part of performing any work one performs rites for the patron deities, geni locii, and other appropriate deities. Naturally enough, the daily routine is also performed before the lararium.

Dies religiosi (vitiosi): like dies atri, but less bad:

no private religious rites may be performed, but public rites may, subject to the same exclusions as for dies atri;

try to avoid making journeys, starting new projects, or doing anything risky.

Nundinae and nundinal letters

Nundinae were originally market-days, held every eighth day, on which Romans came into the city to trade and do business. On Roman calendars the days were given nundinal letters (A to H) to help people see when the next market-day would be. The markets were held on a different day each year.